During the past year detailed epidemiologic evaluation of 509 patients with HAA-positive viral hepatitis has been conducted. Both modes of transmission of the disease and frequency of transmission to household and other contacts have been investigated. The epidemiology of type d and y antigen subdeterminants has also been investigated. During the coming year an additional 500-750 patients will be included in the study. Sera for household contacts of patients already in the study as well as those newly included will be tested by passive hemagglutination (PHA) methods.